The present invention relates to a process for the production of di- and polyamines of the diphenylmethane series (MDA) by reaction of aniline with formaldehyde, in which the mixture of aniline and formaldehyde required for production is produced in the volume ratio required for MDA production simultaneously from the starting compounds nitrobenzene and methanol.
Aniline and formaldehyde are important intermediate products for the polymer industry among other things. Aniline and formaldehyde are used together, for example, as starting materials for the production of methylene diphenyl diamine (MDA) and the corresponding polyamines, and methylene diphenyl diisocyanate (MDI) and the corresponding polyisocyanates, an important monomer for the production of polyurethane. There are a number of processes for the production of aniline and formalin respectively, some of which have been used industrially. Aniline is currently produced industrially by the catalytic gas phase hydrogenation of nitrobenzene with hydrogen in adiabatic (Hydrocarbon Process 59 (Nov. 1979) no. 11, 136; U.S. Pat. No. 3,636,152) or isothermal operation (U.S. Pat. No. 4,265,834) using a Cu or Pd catalyst. The reduction of nitrobenzene with iron (Bechamp process, Winnacker-Küchler Chemische Technologie, 3rd ed., Vol. 4, pp. 170-171) and the heterogeneously-catalyzed gas phase ammonolysis of phenol (Halcon process, U.S. Pat. No. 3,272,865) are of secondary importance.
The production of formaldehyde on an industrial scale is currently carried out substantially by silver-catalyzed dehydrogenation processes (DE-A-2 322 757, U.S. Pat. No. 2,519,788) and the so-called Formox process (GB-A-1 080 508).
In the silver-catalyzed process, methanol is dehydrogenated by air at >600° C. on a silver catalyst with the formation of formaldehyde and hydrogen, the hydrogen being converted to water with atmospheric oxygen in the further course of the reaction or in subsequent reaction stages for the purposes of energy production. The Formox process comprises a two-stage oxidation of methanol to formaldehyde and water (oxidation-reduction cycle of the catalyst), which takes place at lower temperatures in the range 270-300° C., as a rule using molybdenum-iron catalysts.
When the above-described processes are used, the aniline and formaldehyde must be produced and processed independently of each other in separate units. For the production of aniline, particularly by the industrially-definitive hydrogenation process, hydrogen must also be used as a cost-intensive reducing agent.
For the production of methylene diphenyl diisocyanate and the corresponding polyisocyanates (MDI), MDA is reacted with phosgene. The phosgene required for phosgenation is conventionally produced by an industrial process in which carbon monoxide and chlorine are fed over activated charcoal and reacted in the process. The reaction is highly exothermic. Conventionally, a cooled tube bundle reactor is used, the tubes of which are packed with granulated activated charcoal. The temperature of the activated charcoal bed in the reaction zone is about 400° C. and drops by cooling along the tubes to 40-150° C. It is also possible to carry out the reaction in two stages, the first being carried out at a high temperature (200-400° C.) and the second at a lower temperature (40-150° C.). In the majority of applications, the lowest possible residual chlorine content is desirable. Carbon monoxide is thus conventionally used in stoichiometric excess. The reaction is carried out at atmospheric pressure. The gaseous phosgene formed is absorbed in solvent in a further process step. This solution is then used in the production of the isocyanates by reaction with di- and/or polyamines.
For the production of MDA by acid-catalyzed reaction of aniline and formaldehyde, it would be advantageous to produce aniline and formaldehyde simultaneously in a single process, so that fewer unit parts are required and the process is simplified. Furthermore, it would be advantageous from an economic and process safety point of view to replace the hydrogenation hydrogen in the nitrobenzene reduction to aniline with a source of hydrogen that is cheaper and easier to handle, and which also transfers/hydrogen under formation of a useful material.
It has been found that the objects described above can be achieved by the catalytic transfer hydrogenation of nitrobenzene and methanol according to equation (I)

However, a mixture produced according to equation (I) has a molar ratio of aniline to formaldehyde of 1:3, while mixtures with a molar ratio of aniline to formaldehyde of 1.5:1 to 10:1 (correspondingly 1:0.1 to 1:0.67) are conventionally used for industrial synthesis of MDA. Aniline is conventionally not fully converted in this process, so that part of the aniline used can be re-circulated after the reaction. This re-circulated part of the aniline is conventionally supplemented with fresh aniline and converted again with formaldehyde to MDA. A molar ratio of 0.9:1 to 2.4:1 (correspondingly 1:1.1 to 1:0.42) is set industrially for the fresh portions of the aniline and the formaldehyde used in the reaction.
Product mixtures that are obtained according to equation (I) should therefore be supplemented with a significant additional quantity of aniline that has been produced by a conventional process, so that these mixtures are suitable for industrial MDA synthesis. However, this significantly reduces the economy of the process, as conventional nitrobenzene hydrogenation also remains necessary in addition to transfer hydrogenation according to equation (I).